Luminous Buried AGNs in Ultraluminous Infrared Galaxies and the Origin of Galaxy Downsizing

We present the results of our systematic infrared 2.5– 35 μm low-resolution spectroscopy of nearby ultraluminous infrared galaxies (ULIRGs) at z < 0.3, using Subaru, AKARI, and Spitzer. We detected the signatures of optically-elusive, but intrinsically luminous, buried AGNs in a signiﬁcant fraction of ULIRGs classiﬁed optically as non-Seyferts. We found that the energetic importance of AGNs increases with increasing galaxy infrared luminosity, suggesting that the AGN-starburst connections are luminosity dependent. This result may be related to the stronger AGN feedback scenario in currently more massive galaxy systems, as the origin of the galaxy downsizing phenomenon. It is crucially important for SPICA to investigate the AGN-starburst connections in the early universe at z > 1, when both star-formation and AGN activity show peaks.